Adventure mode from Diablo 3. (Beat the game once and unlock the ability to play any mission with this or any character you make)
In addition to this. Reach the level cap and unlock the Paragon system (which is a MUCH better system than BAR).
This will give the next title so much longevity, replay value, end game and infinite progression. Please, someone at gearbox, go have 3 or 4 of your guys go play Diablo 3 until they are paragon 200, have gone through bounties, rifts, greater rifts, adventure mode’s flexibility regarding the story, and you name it.
Gearbox, if you want to make something better than Blizzard and Bungie, just think about it.